Method of protecting metal surfaces



Patented Get 13, 1931 UNITED STATES "PATENT OFFICE,

SUHNEBBEDWAY mm, or wrLun'rrn, rumors, ABSIGNOR 'rownsrnnnnnncrmc couPANY, moonronn'rnn, or nnwvoax, N. 12., A conronnnon' or mrw YORK IETHOD OI PROTECTING METAL SURFACES R o Drawing."

This invention relates to a method of protecting metal surfaces and is related to my =copendin application, Serial No. 227,060, filed Octo er 18, 1927.

r; The object of the invention'is to provide a simple, effective and economical method of rendering metal surfaces capable of resisting the action of moisture or other influences tending to corrode such surfaces. In accordance with the general features of the invention, an article having a metal surface which it is desired to protect is connected as a. cathode in an electrolytic cell in which the bath em loyed contains a saltof an acid containing chromium, such as sodium chromate or potassiumdichromate, as an elecr trolyte. An initial electric current of sufliciently' low cathode current densityto insure substantially no metallic chromium deposition is passed through the cell for a short period of time after which the article is removed' from the cell and dried. vIf desired, the surface of the article which has been subjected to the above described treatment may 5 be readily coated with a protectivepaint, such as a japan, in order to give additional protection and to give the surface a desired finish. It has been found by experiment that when such metals as iron, zinc, cadmium, copper, silver, etc. are connected as cathodes in an electrolytic cell in which the ath contains a salt of an acid containing chromium, such. as sodium chromate or potassium dichromate, and when a current-of sufficiently low cathode current density toprovide conditions under which-metallic chromium is not deposited, is passed through the cell for a short period of time the surfaces of the articles are rendered P ssive to the extent that they will'resistthe corrosive action of atmos heric agents for a muchlonger period of time than surfaces l which have not'been so treated. The sur faces rendered passive are also quite resistant to the action-of sulphur, acids, such as nitric acid andhydrogen sulphide, and numerous other corrosive substances even after the treated articles have been subjected to abrasion. This phenomenon provides a very effective method for protecting metal sur.-

Application filed October 18, 1927.. Serial No. 227,061.

faces and it constitutes the subject matter of the present invention.

Although it is to be understood that the method of protecting metal surfacesforming P-as the subject matter of this invention is a plicable to numerous metals and that the ingredients and conditions. under which the method ispracticedmay be varied within a Wide range, a few specific embodiments of the invention will be described'in detail in order to enable persons "skilled in the art to practice the invention.

One embodiment of the invention consists in a, method for protecting the surfaces of articles made of iron. In this embodiment of the invention an electrolytic cell is employed in which the bath comprises a water solution containing 31 grams per liter of sodium chromate in which is connected an anode of any suitable insoluble material, such as iron, platinum or lead. The article to be treated is connected as a cathode in the above described electrolytic cell, an electric current of a cath-v ode current density of 50 to 100 amperes per square footds passed through the cell for a short period of time, and the temperature of the bath is maintained at room temperature (2025 C.) during the passa e of the-cur .rent. For rendering the sur aces resistant to the action of atmosphericagents 5 to 15 minutes has proven to be a satisfactory length of time for the electrol ic treatment. After this treatment the artic e is removed from the electrolytic cell-and dried in an suitable manner. An iron article so treate will be found to have .a greatly increased resistance toatmospheric corrosion and will not rust to any great extent unless exposed toextreme conditions for a long periodlof time.

Another very practical use to which the process may be put is in'the. protection of,

articles coated with other metals such as zincor cadmium. It has been attempted heretofore in some instances to coat articles, which it is desired to protect, with a coating of zinc, since zinc is resistant 'to corrosive agents, and then to apply a coatingof a protective paint, such as a japan,,upon thezinc coating.

t has been found, however, that-the 'zinc' in the first coating has a tendencyto react with the oleagenous fatty acids, such as oleic or palmitic acid, present in the protective paint to form zinc soaps which will prevent the adherence of the protective paint to the zinc coating with the result that the protective paint has a tendency to scale off of the zinc coating after a short period of time. In this embodiment of the invention, the article to be protected is first coated with another metal such as zinc or cadmium and the coated article is then connected as a cathode in an electrolytic cell of the type described above inwhich the bath comprises a water solution containing 65 grams per liter of potassium dichromate and an electric current of low cathode current density ispassed through the cell for a short period of time while the bath is maintained at room temperature. The article is then removed from the electrolytic cell and dried. It will be found that after such treatment the metallic coating upon the article has been rendered passive in the same manneras the iron article hereinbefore described, and an article so treated will offer great resistance to corrosion. If desired, the article treated in the manner just described may be coated with a protective paint, such as a japan, in order to give additional protection to the article and to provide a desired finish. When the article is protected in this manner the surface of the zinc or cadmium which has been subjected to the above outlined treatment does not react with the fatty acids in the paint to form metallic soaps and as a consequence the paint adheres firmly to the zinc coating even when subjected to very adverse. conditions.

It is believed that the embodiments of the invention just described will sufiiciently illustrate how the invention may be practiced, but it is, of course, to be understood that the invention is not limited to the specific examples given hereinbefore. The invention may be practiced with satisfactory results upon metals other than iron, zinc and cadmium, such as copper and silver, and also the conditions under which the articles are rendered passive may be varied within a wide range. For example, the bath included in the electrolytic cell need not necessarily be composed of a water solution containing 31 grams per liter of sodium chromate or 65 grams per liter of potassium dichromate, but the amounts of these ingredients present in the baths may be varied within wide limits and still satisfactory results may be obtained.

Nor does the bath need to contain only sodium chromate or potassium dichromate in water solution, but additional conducting materials may be present in order to vary the operating conditions of the bath. Satisfactory results may also be obtained when other alkali salts of chromic or dichromic acids are employed and such materials as potassium, ammonium and lithium chromates and sodium, ammonium and lithium dichrom'ates may be substituted f" th' above described materials in order to render metal surfaces resistant to corrosive agents. Also the amount of current flowing through the cell may be varied over a wide range so long as the initial cathode current density is maintained below the range within which metallic chromium would be deposited from the particular bat-h used in quantities more than traces. The temperature of the bath may also be varied to provide the optimum conditions of operation obtainable with the particular bath and current density employed, and also the time of treatment may be varied. Satisfactory results may be usually obtained, however, when the bath is operated at room temperature (20 to 25 C.) and the article is treated for from three to fifteen minutes in the electrolytic cell.

A satisfactory theory of the reactions which take place during the above described operations has not yet been developed, but

some or all of a number of changes may occur. For example, the surface of the metal which it is desired to protect may be oxidized or otherwise chemically changed. Or a physical change or displacement may be effected in the molecules comprising the surface layers of the metal, so that the surface of the metal is rendered passive. vOr it may be possible that the metal surface is coated with an extremely thin film composed of a compound or compounds derived from the electrolytic bath. Whatever the changes or reactions which occur may be, a treatment embodying the invention renders the surface of the treated metal resistant to corrosion, and at the same time the weight and dimensions of the treated article remain substantially unchanged. Also the appearance of the article remains practically unchanged in most instances after treatment. For example, copper articles originally having bright, clean surfaces usually exhibit a similar appearance with possibly a slight irridescence being noticeable after having been treated in accordance with the invention, but there is no evidence that the articles are coated with any colored or colloidal material of the types some times found in electroplating baths for the deposition of metallic chromium.

The method described above provides an extremely simple and inexpensive means whereby surfaces of metal articles may be protected against corrosion and abrasion Without any material variation in the weight or dimensions of the article. The treatment is especially effective in treating irregular shaped articles for not only the plane surfaces of the articles are protected, but the protective effects are also obtained in corners, crevices, and the walls of holes in the articles.

By the statement employed hereinbefore in the specification and also in the appended claims, that the cathode current density is.

to an electric current of a cathode current density below the range in which chromium is deposited for from 5 to 15 minutes and maintaining the bath at room temperature.

2. A process for protecting surfaces of metal articles, which consists in coating the surface of an article with a metal of the zinc group and subjecting the article as a cathode in a bath including a salt containing chromium to an electric current of a cathode current density below the range within which chromium is deposited, and coating the treated article with a protective paint.

3. A process. for protecting surfaces of metal articles, which consists in coating the surface of a metal article with zinc, subjecting the coated article as a cathode in a bath including an alkali salt of an acid containing chromium to an electric current of a cathode current density below the range within which chromium is deposited, and coating the treated article with a protective paint.

4. A process for protecting surfaces of rent density below the range in which chromium is deposited.

7. A process for protecting surfaces of metal articles which consists in coating the surface of an article with zinc, subjecting the article as a cathode in a bath containing about grams of potassium dichromate per liter to an electric current of a cathode current density below the range in which chromium is deposited, and covering the thus treated article with a protective paint.

In witness whereof, I hereunto subscribe my name this 30th day of September, D.

SUMNER REDWAY MASON.

metal articles, which consists in coating the surface of an article with another metal, sub

jecting the coated article as a cathode in a bath containing a-dichromate to an electric current of a cathode current density below the range within which chromium is deposited, and coating the treated article with a protective paint.

5. A process for protecting surfaces of metal articles, which consists in coating the surfaces of an article with zinc, subjecting the article as a cathode in a bath containing 65 grams of potassium dichromate per liter to an electric current of a cathode current density below the range within which chromium is deposited for from 5 to '15 minutes, maintaining the bath at room temperature, removing the article from the bath,

drying the article and coating the treated article with a protective paint.

6. A process for protecting surfaces of metal articles which consists in subjecting the article as a cathode in a bath containing about 65 grams of potassium dichromate per liter to an electriccurrent of a cathode our- 

